Gear differentials include compound planetary gear sets carried within a housing for interconnecting a pair of output shafts. The planetary gear sets permit the output shafts to rotate in opposite directions with respect to the housing. An input shaft is operatively connected to the housing for rotating the housing about a common axis of the output shafts.
Sun gear members of the planetary gear sets, also referred to as "side" gears, are coupled to inner ends of the output shafts. Planet gear members of the same sets, also referred to as "element" gears, operatively connect the side gears for relative rotation in opposite directions. The sun gear members rotate together with the output shafts about the common axis of the output shafts. However, the planet gears rotate about axes that can be variously offset and inclined with respect to the common axis.
One known type of gear differential, referred to as a "parallel-axis" gear differential, includes the sun and planet gears mounted about axes that extend parallel to each other. The planet gears of this type of differential are generally mounted in pairs within the housing. One portion of each planet gear meshes with one of the side gears, and another portion of each planet gear meshes with its paired planet gear.
The planet gears are individually supported for rotation on shafts or within pockets formed in the housing. The shafts are journalled in bores formed in the housing at opposite ends of the planet gears. The pockets provide bearings for slidably supporting outside cylindrical surfaces of the planet gears formed by top lands of the planet gear teeth.
One example of a parallel-axis gear differential having planet gears individually supported within housing pockets is disclosed in U.S. Pat. No. 3,706,239 (MYERS). The pockets, together with other gear mounting surfaces in the differential of Myers, provide frictional surfaces for opposing relative rotation (i.e., differentiation) of the planet gear set. The amount of friction is proportional to the total drive torque transmitted by the differential housing, and the friction is used to support torque differences between the output shafts.
Differentials that develop a frictional resistance to differentiation proportional to drive torque (like the one disclosed in Myers) are referred to as "torque proportioning" differentials. The frictional resistance helps to compensate for an uneven traction condition presented to a pair of drive wheels by delivering more drive torque to the wheel with better traction. In turns, more drive torque is delivered to the slower rotating drive wheel.
Gear tooth forces acting at two different locations on the planet gears in Myers tend to misalign or tilt the planet gears within their pockets. Although some misalignment of the planet gears can be used to generate increased frictional resistance to differentiation, a spacer is required to provide additional radial support for the planet gears. The spacer includes arcuate segments for enclosing openings in the pockets between the side gears.
Another example of a parallel-axis gear differential having planet gears mounted within housing pockets is disclosed in U.S. Pat. No. 5,122,101 (TSENG), a patent commonly assigned herewith. The planet gears are formed as so-called "combination" gears having two gear sections separated by a stem. A first of the gear sections of each combination gear includes respective points of mesh with one of the side gears and with a second of the gear sections of a paired combination gear. The second section of each combination gear includes a point of mesh with the first portion of its paired combination gear. The two points of mesh between paired combination gears straddle points of mesh between the paired combination gears and the side gears along the common axis of the side gears.
Although the two points of mesh between paired combination gears reduce the tendency of the combination gears to tilt within their pockets, movement of the combination gears toward the common axis can produce interference with teeth of the side gears. The interference can be caused by either angular or rectilinear gear movements that reduce backlash between the combination gears and the side gears. The loss of backlash can cause the mating gear teeth of the gears to bind together, thereby increasing gear tooth wear and possibilities for gear tooth failure and producing inconsistent differential performance.